The present invention relates to printing element drive devices such as a thermal printing head (thermal head) and a LED printing head.
Among the drive methods in a printing element drive device such as a thermal head is a latch-less control type drive method. This type of drive method uses, for instance, a drive circuit as shown in FIG. 1 consisting of a shift register 2 having n memory cells D.sub.1 to D.sub.n for n respective heating elements 1.sub.1 to 1.sub.n and drive elements 3.sub.1 to 3.sub.n that receive respective memory cell outputs of the shift register 2 and a strobe signal STB1 (inverted) or STB2 (inverted). In this drive circuit, at first serial printing data DI of n bits are stored into the shift register 2 while n pulses of a clock signal CLK are applied to the shift register 2. Then, while the strobe signal STB1 is in an on-state, the drive elements 3.sub.1 to 3.sub.n/2 operate to drive the heating elements 1.sub.1 to 1.sub.n/2. Then, while the strobe signal STB2 is in an on-state, the drive elements 3.sub.n/2+1 to 3.sub.n operate to drive the heating elements 1.sub.n/2+1 to 1.sub.n .
As shown in FIG. 2, in the conventional latch-less control type drive method, a data transfer period is provided outside the periods in which the strobe signal STB1 or STB2 is in an on-state. This is because if the printing data is transferred during the strobe signal on-period, the heating dots change in the midst of printing to prohibit a normal printing operation.
The one printing period SLT including the data transfer period is usually set at about 10 msec. To provide the data transfer period outside the strobe signal on-periods under the restriction of this printing period, there is no other measure than shortening the strobe signal on-periods. However, to assure sufficient printing density, it is much desired to avoid the shortening of the strobe signal on-periods. On the other hand, if the transfer rate is increased (for instance to 4 MHz) from the conventional case (for instance 1 MHz), noise problems may occur.
The above-mentioned number n takes such values as 1,056 and 2,048, and the shift register 2 and the drive elements 3.sub.1 to 3.sub.n are constituted of a plurality of IC chips of 64 or 96 bits. Therefore, it is not always the case that the number n (number of dots of the thermal head to be driven) is not divided by the number N of dots of the IC chips used. Conventionally, where the use of m IC chips causes a fraction and (m-1) IC chips are insufficient for the n dots, the shift register 2 having mN memory bit cells is formed by m IC chips, and (mN-n) fractional cells serve to store dummy bits that are not related to the printing in which the associated drive elements are not connected to any heating elements (non-contact drive elements), as shown in FIG. 3.
However, the non-contact drive elements are provided only in the IC chips associated with only one of the two strobe signals, drive currents when the strobe signal STB1 is applied differ from those when the strobe signal STB2 is applied. This will cause unevenness in the printing density distribution.